If this is your only travel and you book well in advance, I guess that normal tickets will be cheaper than Interrail and reservation cost.

On the page in the link you have information how to make reservations. I suggest you call VR, the Finnisch railway company. 

https://www.interrail.eu/en/plan-your-trip/tips-and-tricks/trains-europe/trains-country/trains-finland

Am in Finland just now and a couple of days ago phoned the VR number in that link to book seat reservations. It was easy to speak to someone  and the seat reservations were both emailed to me and sent by sms text so no need to collect from the station. We have not found any manned railway stations in Finland  apart from Helsinki main train station. The Eurail information about collecting reservations from stations is out of date in my view. 

Was in FI myself last spring-not that long (visited Nokia also) and found that only-beside HEL a very few big stations, like Tampere till have counters with staff. But even in HEL this has very limited opening hrs. For Scandinavia (though yes, I know strictly speaking Fi is not that-but very much comparable to it)  most stations have a convenience store (7-11 in DK, Pressbyran in SE, R-=rautatie=rails-Kioskki in FI) and THERE, if the teenager working has got some instructions, they can do access toVR-systems and organise some things. Comparable to how in some other countries f.e. post-office or lottery shops have access to system and can help people also with tickets etc.

But for this (I was also told one does not need to RES with pass-with the chance that at any stop someone can some and chase you away as tey have the RES for that) like sleeper etc-one has to call the call-centre. As is the same in NO with EnTur.

The Santa Express is a holiday-themed train ride that is typically offered by various train operators during the Christmas season. The exact details of how to purchase a ticket for the Santa Express can vary depending on the operator and the location. Here are some general steps that might help you buy a ticket:

1. Determine the operator: Research and find out which train operator is offering the Santa Express in your area.

2. Check the schedule: Check the dates and times of the Santa Express and make sure it fits with your schedule.

3. Visit the operator's website: Most train operators have an online ticket purchasing system. Visit the operator's website and look for information on the Santa Express, including ticket prices and availability.

4. Choose your tickets: Select the date, time, and number of tickets you need and proceed to the checkout.

5. Make payment: Complete the payment process using a credit card or other accepted method of payment.

6. Confirm your booking: After you have made payment, you will receive a confirmation of your booking and a ticket for the Santa Express.

It is recommended to book your tickets as early as possible, as the Santa Express is a popular event and tickets can sell out quickly.

That may be the case but it is in any case the nickname of a regular night train between Helsinki and Rovaniemi - Kemijärvi:

https://www.eurail.com/en/get-inspired/trains-europe/night-trains/santa-claus-express

Hello @NewcastleTourists and everyone,

It has been flagged and reported to the relevant team. Lately, we have received the email from VR that their current and upcoming system on the reservation will only work on an e-mail basis.

Collecting reservations at a Finnish train station does not and will not apply anymore, and the SMS messages also will be soon outdated. The information about it on our website will be also updated.

Cheers!